The present invention relates an electrical storage capacitor system consisting of capacitors connected in series.
Where large-capacity capacitors are combined to constitute an electrical storage unit, if the capacitors are connected in series, uniform voltage must be assigned to the capacitors. We have already proposed an electrical storage system, referred to as an energy capacitor system (ECS), using electric double-layer capacitors. In this energy capacitor system, parallel monitors for monitoring and controlling voltages are connected with the capacitors connected in series. This permits maximum charging within the range of breakdown voltages of the capacitors.
FIG. 8 shows an example of the circuit configuration of such a parallel monitor. FIGS. 9a and 9b are graphs illustrating the charging/discharging characteristics of the prior art circuit shown in FIG. 8 when and after the circuit is initialized to its initial state.
In a parallel monitor, as shown in FIG. 8, a comparator CMP compares the voltage developed across a capacitor C with a reference voltage Vr. If this compared voltage reaches a set value determined by the reference voltage Vr, a transistor Tr is turned on, thus bypassing a charging current. As the charging operation progresses, the voltage developed across the capacitor C rises but is then kept at the set value as indicated by (1) in FIG. 9a. With respect to other series connected capacitors having larger capacitance, the voltages rise more slowly and reach full charge at point (2). After reaching full charge, relaxation charging is performed with a constant voltage until a next discharging (3) is commenced.
In this way, the prior art energy capacitor system (ECS) is initialized to its initial state such that the voltages developed across the capacitors become equal to the upper limit of voltage (full charge voltage) at point (1). Then, each capacitor is started to be discharged and charged from this initial state.
The above-described structure is simple and economical to realize. Also, the operation is reliable. Therefore, this structure has played a great role in putting the energy capacitor system into practical use. However, during the period (1) to (2), while the parallel monitor is turned on the charging current is bypassed by at least one capacitor. During this period, the charging energy is wasted as heat, thus constituting a problem. In particular, in the parallel monitor shown in FIG. 8, the transistor Tr is driven into conduction when the set voltage at (1) is reached. This forms a bypass circuit. The voltage is clamped to prevent the voltage from rising further. Therefore, the bypass circuit generates loss, or heat, corresponding to the charging currentxc3x97full charge voltage.
The heat generated during the period (1) to (2) is produced every charging/discharging cycle if the device is a secondary battery. On the other hand, a capacitor produces heat only once on initializing. After the initializing, the voltage varies from the set, full charge voltage, and discharging is done downwardly as indicated by (4). If charging is resumed, all the capacitors are almost simultaneously fully charged again with the original voltage (5) and so little bypass current flows. Strictly speaking, the capacitors reach full charge with slight time differences because of slight characteristic variations and leakage currents during use. During the slight time differences, a bypass current flows. Consequently, loss takes place after the initializing operation for initializing the system to its initial state.
On the other hand, where capacitors connected in series and having no parallel monitors are charged, the voltage developed across each capacitor varies as indicated by FIG. 9b. That is, when the charging operation is stopped, the voltage developed across a lower capacitance capacitor shows a higher value. Variations in capacitance among the individual capacitors result in different terminal voltages among the capacitors. Furthermore, terminal voltage differences among the capacitors produced when charging may result from different residual voltages of the capacitors when the charging is stopped.
In the energy capacitor system (ECS), the terminal voltages of the capacitors are all set to the upper limit value at instant (2). That is, the terminal voltages of the capacitors are clamped to the upper limit value to initialize themselves into their initial state. Once this initializing operation is done, all the capacitors can be charged and discharged from the upper limit value with uniformly assigned voltage.
It is an object of the present invention to provide an electrical storage capacitor system which is equipped with parallel monitors having an excellent initializing function and which can efficiently initialize itself to its initial state during periods other than the charging/discharging cycles normally used without providing any special initializing cycles.
To achieve this object, the invention provides an electrical storage capacitor system having capacitors connected in series to store electricity, the capacitor system being characterized by having a variation decision means, an operation decision means, and an initializing charging means. The variation decision means judges variations in amount of charge among the capacitors. The operation decision means judges whether the capacitors are being charged or discharged. The initializing charging means charges the capacitors to return them to their initial state according to the results of the decisions made by the variation decision means and by the operation decision means.
The variation decision means judges variations in amount of charge among the capacitors according to the full charge voltage obtained when any of the capacitors reaches a given charge voltage as a result of charging.
The operation decision means can make a decision whether the capacitors are being charged or discharged with a large current. In addition, the operation decision means can make a decision as to whether the charge level is within a given tolerable level.
Other objects and features of the invention will appear in the course of the description thereof, which follows.